Re: [PATCH v12 6/8] mm: support reporting free page blocks

From: Michal Hocko <mhocko@kernel.org>
To: "Wang, Wei W" <wei.w.wang@intel.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
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Subject: Re: [PATCH v12 6/8] mm: support reporting free page blocks
Date: Tue, 25 Jul 2017 16:53:34 +0200	[thread overview]
Message-ID: <20170725145333.GK26723@dhcp22.suse.cz> (raw)
In-Reply-To: <286AC319A985734F985F78AFA26841F739283F62@shsmsx102.ccr.corp.intel.com>

On Tue 25-07-17 14:47:16, Wang, Wei W wrote:
> On Tuesday, July 25, 2017 8:42 PM, hal Hocko wrote:
> > On Tue 25-07-17 19:56:24, Wei Wang wrote:
> > > On 07/25/2017 07:25 PM, Michal Hocko wrote:
> > > >On Tue 25-07-17 17:32:00, Wei Wang wrote:
> > > >>On 07/24/2017 05:00 PM, Michal Hocko wrote:
> > > >>>On Wed 19-07-17 20:01:18, Wei Wang wrote:
> > > >>>>On 07/19/2017 04:13 PM, Michal Hocko wrote:
> > > >>>[...
> > > We don't need to do the pfn walk in the guest kernel. When the API
> > > reports, for example, a 2MB free page block, the API caller offers to
> > > the hypervisor the base address of the page block, and size=2MB, to
> > > the hypervisor.
> > 
> > So you want to skip pfn walks by regularly calling into the page allocator to
> > update your bitmap. If that is the case then would an API that would allow you
> > to update your bitmap via a callback be s sufficient? Something like
> > 	void walk_free_mem(int node, int min_order,
> > 			void (*visit)(unsigned long pfn, unsigned long nr_pages))
> > 
> > The function will call the given callback for each free memory block on the given
> > node starting from the given min_order. The callback will be strictly an atomic
> > and very light context. You can update your bitmap from there.
> 
> I would need to introduce more about the background here:
> The hypervisor and the guest live in their own address space. The hypervisor's bitmap
> isn't seen by the guest. I think we also wouldn't be able to give a callback function 
> from the hypervisor to the guest in this case.

How did you plan to use your original API which export struct page array
then?

> > This would address my main concern that the allocator internals would get
> > outside of the allocator proper. 
> 
> What issue would it have to expose the internal, for_each_zone()?

zone is a MM internal concept. No code outside of the MM proper should
really care about zones. 
-- 
Michal Hocko
SUSE Labs

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